I have a timeline of clips that I want to put into a different order. I can select some clips, and drag them to a different place while holding the Option key. My understanding is that this should rearrange the rest of the clips in the timeline (moving them either forward or backward) so that the group of clips I'm moving ends up in a different place.

However, what seems to happen is that it will either leave a gap where the clips were originally, or I'll end up with two copies of them.

How do I just move clips around in the timeline (in other words, change the order) without making copies or leaving gaps?